1. Field of the Invention
The present invention relates generally to optical measuring equipment. More particularly, the present invention relates to a system and method for automatically measuring the modulation transfer function of an optical system.
2. Background Information
An electro-optical imaging sensor is a device that converts radiant energy of a particular wavelength or range of wavelengths into an electrical signal. The capability of the electro-optical sensor to resolve the details of an object is referred to as its “resolution.” A useful measure of the resolution of the electro-optical sensor is provided by a modulation transfer function (“MTF”).
The MTF of an electro-optical sensor is a measure of the ability of the sensor to resolve fine detail of a scene. In other words, the MTF expresses the ability of an optical or electronic device to transfer signals faithfully as a function of the spatial or temporal frequency of the signal. MTF is analogous to the sine wave frequency response of an RC filter. However, a sensor MTF can include the effects of the optics, detector, electronics, display, and even the human eye for end-to-end performance predictions.
The MTF is measured by characterizing output amplitude response as a function of input sinusoidal spatial frequency. In other words, it is the ratio of the percentage modulation of a sinusoidal signal leaving to that entering the device over the range of frequencies of interest. A detailed discussion of the MTF and of conventional methods and apparatus for determining it can be found in U.S. Pat. No. 3,743,427, the disclosure of which is hereby incorporated by reference in its entirety.
In the past, knife edge and spot scan techniques have been used to measure the MTF. However, there are a number of drawbacks associated with such conventional methods and arrangements for evaluating the MTF of a device or system. The most common technique for analyzing the MTF of an optical system is the knife edge technique, in which the Fast Fourier Transform of the differentiated edge response is taken. The knife edge technique is described in, for example, U.S. Pat. No. 5,629,766, the disclosure of which is hereby incorporated by reference in its entirety. However, the results of this technique can be easily misinterpreted due to inadequate removal of offset, edge roughness noise and focus. Furthermore, the knife edge technique is very difficult to implement in a staring array due to the limited number of samples across the edge. In addition, the determination of the DC component can produce huge errors if the sensor has significant scattering or large area crosstalk.
Similarly, the spot scan technique is difficult to implement due to the data collection effort required to examine multiple samples of a single pixel. With the spot scan technique, two-dimensional phasing and alignment can present problems, while scattering and crosstalk can become a huge source of error. In addition, both techniques suffer from the difficulty of performing repeatable measurements with certainty. In sum, both of the techniques are time-consuming and cumbersome to implement.
Another method for determining the MTF of an optical system is by computing the contrast transfer function (“CTF”) from a square wave (bar) target. The CTF is the comparison of the input and output intensities of a square wave (bar) target. The CTF, also referred to as the square wave response (“SWR”), is determined by imaging a bar target onto a sensor and determining a normalized curve of peak-to-peak output as a function of spatial frequency. The MTF at desired frequencies can then be calculated as a linear combination of the CTF (SWR) measurements. In the past, a minimum resolvable target (“MRT”) was used for calculating the CTF. Each MRT target is comprised, however, of only one frequency. Therefore, multiple MRT targets would be required if the analysis of multiple frequencies is desired. In addition, conventional MRT targets do not provide an easy means for aligning the targets to the proper orientation for computing the CTF and, consequently, the MTF of the optical system.
Accordingly, it would be desirable to provide systems and methods for measuring the MTF of an optical system which are more accurate, faster, and repeatable, but which do not require the time and complexity of conventional systems and methods.